The goal of this research proposal is to define functional aspects of immunoglobulin molecules at a structural level. More attention will be directed to IgM to specifically define: 1) The structural features of the molecule responsible for its bonding to complement; 2) The nature of sequence hypervariability as examined through the second cyanogen bromide fragment; 3) The nature of micron-chain deletions; 4) The structural basis for J-chain bonding in subunit conformation. Our present data on the order and structure of CNBr fragments of IgM will facilitate the proposed structure-function analyses.